Porous silicon (PS) has a low dielectric constant and high resistivity which make it useful for various applications. Some reported applications include: isolation of power devices from other circuitry and optoelectronics/photoluminesence. PS is formed by anodic oxidation of a silicon wafer in a solution of hydrofluoric acid and methanol. Uniform, thick (100 .mu.m) films having a specular surface can be produced in 15 minutes. The PS film structure consists of interconnected pores in a single crystal silicon matrix which retains the orientation of the silicon wafer. The pore fraction is a function of the anodization conditions and typically ranges from 30% to 50%. The pore structure of PS is determined by the doping level of the starting silicon wafer. In degeneratively doped n-type or p-type silicon, the structure consists of many long pores running perpendicular to the silicon wafer surface with many small "buds" on the sides of the pores which can result in branching. In lightly doped n-type or p-type silicon, the pore structure consists of an apparently random, interconnected distribution of voids. For a pore fraction of 50%, the size of the pores in lightly doped material is in the 50 .ANG. range, while those in degeneratively doped material are around 150 .ANG.. Once the sidewalls of a pore have formed, they are remarkably resistant to further anodic attack. At the start of anodization, inhomogenities in the region of the semiconductor-electrolyte interface result in localization of the current flow and the initiation of pores. The silicon between the pores is depleted and is therefore highly resistive (&gt;10.sup.5 Ohm cm) compared with the electrolyte and the bulk silicon. This results in preferential current flow down the electrolyte in the pores causing the reaction to occur only at the pore tips.
The construction of a commonly employed chamber is illustrated in FIG. 1. The silicon wafer 10 separates the front 12 and rear 14 half cells, each having a platinum electrode 16 and pumped supply of electrolyte 18. The PS film forms on the anode side of the wafer; the silicon is cathodic in the back half cell and does not react. The most important variables affecting the PS structure are (1) substrate doping type and level; (2) HF concentration (10-48%); and (3) current density (typically 1-100 mA/cm.sup.2). For a fixed substrate resistivity, similar film porosity can be produced using various combinations of HF concentration and current density. Porosity generally increases with increasing current density and decreasing HF concentration. Porous silicon has a high internal surface area, but the impurities found on these surfaces (oxygen, carbon, fluorine, and hydrogen) should not represent a significant problem for subsequent front-end processing.
Selective anodization has been demonstrated using various masking materials. Two approaches have been used to define the PS regions: (1) ion irradiation to inhibit anodization and (2) conventional photolithography combined with various masking layers. The use of ion irradiation damages the single crystal silicon regions; this damage must be removed by subsequent processing. For conventional photolithography and masking layers, it is difficult to find a masking material that will not erode in the presence of the HF during the anodization process, which may last on the order of 15 minutes, and that is compatible with standard processing. Several masking materials have been proposed. These include silicon nitride, MOCVD GaAs, RTCVD SiC, and LPCVD SiC. It is not clear whether silicon nitride of reasonable thickness can withstand HF for the duration required for thick porous silicon regions. While GaAs and SiC can withstand HF, the proposed MOCVD GaAs, RTCVD SiC, and LPCVD SiC processes occur at high temperatures (600-1300.degree. C.). These high temperatures present process integration problems. Therefore, a selective anodization process that is compatible with high volume production remains to be demonstrated.